The invention relates generally to microfluidic devices and more particularly to a system and method for sample injection onto a microfluidic capillary electrophoresis channel.
Electrophoretic separation of bio-molecules is very important in modern biology and biotechnology applications such as DNA sequencing, protein molecular weight determination and genetic mapping. Electrophoresis is a process by which individual molecular species are separated in a conductive medium (such as a liquid solution or a cross-linked polymer) by applying an electric field. The charged molecules migrate through the solution and separate into distinct bands due to their mobility difference through the media. The rates are influenced by factors such as a viscosity of the solution, a mass and charge of the molecules, and a strength and duration of the electric field.
An increase in a voltage gradient (V/cm) applied to the electrophoretic device results in a corresponding decrease in the time needed to perform the separation. However, increasing the voltage gradient is governed by certain constraints. For example, increasing the voltage gradient beyond a certain point may result in an increase in joule heating which would in turn alter the properties of the medium in which the molecules are being separated. The change of the medium properties leads to an increase in sample diffusion and thus degraded the separation resolution. In order to overcome the above limitations, electrophoresis can be performed in a capillary or miniaturized channel. The large surface-area-to-volume ratio of the electrophoretic devices offers efficient dissipation of Joule heat, allowing higher electric field to be used, thus resulting in the shorter analysis time and better separation efficiency.
Microfluidic devices are small compact devices that perform chemical and physical operations such as capillary electrophoresis with microscale sample volumes. These devices often have the benefits of fast reactions, rapid detection, small reagent consumption, ease of automation and simple transfer between reaction vessels. Microfluidic devices are commonly referred to as “lab-on-a-chip.”
Typically in microfluidic devices, sample volumes used are very low, thus making it difficult to detect analytes of low concentration. Often, a “preconcentration” step is incorporated into the microfluidic device to increase the sample analyte concentration. However, the typical concentration techniques are not applicable to microfluidic devices due to the small sample size capacity of the device. In addition, concentrating the analyte of interest may also lead to contaminating the analytes, which can interfere with the downstream analysis. For example, in electrokinetic injection, a high concentration of contaminating salt ions will decrease the amount of the charged analyte that is loaded onto the analysis channel.
In field-amplified sample stacking, the sample is dissolved in a low ionic conductivity solution, which in turn contacts a high ionic conductivity solution in the channel, and an electric field causes the sample salts (or ions) to flow into the high ionic conductivity solution, thereby increasing the sample analyte concentration in the low ionic conductivity solution. However, the sample preconcentration may still be insufficient for high optical detection sensitivity.
In filtered sample stacking, the sample flows into a filter such as a porous polymer plug with a pore size that allows small molecules to pass through but blocks larger molecules (such as proteins or DNA). The sample is preconcentrated upstream of the filter. However, the filter is often formed in the channel using complex and cumbersome manufacturing techniques. Furthermore, the filter is permanently attached to the channel, and therefore the microfluidic device must be discarded if the filter clogs, tears or otherwise deteriorates.
Therefore, there is a need for a microfluidic device that provides sample preconcentration, minimizes the effect of concentrating small molecule contaminants, and reproducibly loads a small sample volume onto an analysis channel in a convenient and cost-effective manner.